(1) Field of the Invention
This invention relates generally to current mirrors and relates more particularly to a short-circuit detection using a current mirror configuration.
(2) Description of the Prior Art
Current mirrors are the most widely use analog circuit. Most of the transistors in an analog integrated circuit are parts of current mirrors. A current mirror may be thought of as an adjustable current regulator, the current limit being easily set by a single resistance.
Current mirrors are used as current sources. An ideal current source has infinite output impedance. That is, the output current does not change, even for large swings in output voltage. This means that current mirrors have very high impedance.
A challenge to the designers of electronic circuits is to find a low-cost solution to detect short-circuits of IC devices. This is especially important for diagnostic and safety circuits. There are various patents in regard of this area.
(U.S. Pat. No. 6,683,765 to Kanamori) describes a switching power unit performing an over-current protecting operation by shortening a switching pulse, wherein an oscillation frequency changing circuit drops an oscillation frequency of an oscillator when an over-current is detected by an over-current detection circuit, and an RS flip-flop circuit performs the over-current protecting operation. When an output voltage is dropped due to a short circuit etc., the oscillation frequency changing circuit further drops the oscillation frequency of the oscillator. In an arrangement in which a frequency under a normal loading condition is made higher by considering the response delay in the over-current protecting operation in this manner, an over-current detection output is given to the oscillation frequency dropping circuit via the RS flip-flop circuit, so that it is necessary to provide a time constant circuit which prevents hunting and it is possible to miniaturize chip size of an integrated circuit itself. As a result, in the switching power unit for performing the over-current protecting operation, it is possible to miniaturize externally provided parts by making a switching frequency higher and to miniaturize the integrated circuit itself.
(U.S. Pat. No. 6,657,877 to Kashima et al.) discloses a power supply circuit improving the power factor when the load is light, and incorporates a starting circuit for reacting quickly to the changes in the output from the power supply circuit and a short-circuit detecting means for detecting the short-circuit of the feedback signal. The apparatus includes: an error amplifier; a comparator that monitors the output from the error amplifier and generates an offset regulating current I.sub.SO; a multiplier; a sensing current comparator that compares the output signal from the multiplier and the AC line current and generates a reset signal; a timer that directly monitors the inputted zero-cross signal; and a comparator for short-circuit detection, that facilitates reducing the exterior parts and components.
(U.S. Pat. No. 6,680,835 to Tabata) describes a semiconductor device comprising a short circuit protecting system capable of enhancing the detection precision of a collector current, thereby carrying out a reliable short circuit protection. An IGBT (1) having a collector (C) connected to a terminal (T1) and an emitter (E) connected to a terminal (T2) is provided, and has a sense emitter (SE) connected to a terminal (T2) through a variable resistor (VR1) to be a current and voltage converting section. A sense potential is output from an end on the sense emitter (SE) side of the variable resistor (VR1) and is given to a terminal (T11) of a current ratio detecting section (15). A gate of the IGBT (1) is connected to a terminal (T3) and an output of the current ratio detecting section (15) is connected to a terminal (T4).